In U.S. Pat. No. 3,784,377 issued January 8, 1974 to Marilyn Levy and Richard G. LeSchander, for "Curve Analysis Method In Color Printing" there is disclosed and claimed a method whereby the clear, unexposed portion of the negative can be used instead of a grey card image to predict the filter pack required to obtain good color balance in the print. The difficulty with the U.S. Pat. No. 3,784,377 method is that it is valid only when the scene is illuminated by light of a very limited range of color temperatures. Exposures made at other color temperatures require additional printing filter corrections.
In U.S. patent application Ser. No. 555,323, filed Mar. 4, 1975, now U.S. Pat. No. 3,953,135 of Marilyn Levy and Milan Schwartz for "Curve Analysis Method In Color Photography" and assigned to a common assignee and of which this application was copending, there is disclosed and claimed a method of approximating the characteristic curves of a color film exposed at a variety of color temperatures where the characteristic curve of a film exposed at one color temperature is known.
The method involves the use of a linear curve which relates the .DELTA. log E change of blue and green exposure with change of color temperature. This linear curve is derived experimentally from the characteristic curves obtained by exposing several color negative films to a sensitometer, including a step tablet and a means for varying color temperatures. The films are exposed at color temperatures ranging from 2900.degree. K. to 6700.degree. K. and then processed in a color developer. Transmission densities of the developed film to red, green, and blue light are determined with a densitometer and the results plotted as a curve relating red, blue, and green density to relative log exposure. These characteristic curves are then analyzed by placing them over the curves obtained at 6700.degree. K. so that the red curves are superimposed. The change in blue and green exposure with color temperature for each set of curves is determined graphically along each curve. The .DELTA. log E determinations are averaged and the results plotted as a linear curve relating the .DELTA. log E change of blue and green exposure from 6700.degree. K. to 2900.degree. K. This curve can be plotted as .DELTA. log E vs. color temperature or Mired Value, which is a value equal to 1,000,000/color temperature. This linear curve can then be used to determine the characteristic curve of an unknown color film exposed to any color temperature providing the characteristic curve for one color temperature is determined experimentally. This is done by determining from the linear curve the .DELTA. log E change for the blue exposure and the green exposure from the experimental color temperature to the new color temperature. The change is plotted on the characteristic curve of the known temperature to produce the predicted points for the unknown curves.